Sensored brushless motor technology is well-known and is useful for minimal flaw control at low speeds and reliable rotation. A sensored system has one or more sensors that continuously communicate with a motor controller, indicating to it what position the rotor is in, how fast it is turning, and whether it is going forward or reverse. In trapezoidal drive, position of the rotor is determined by sextant. FIGS. 1A and 1B are representations of the six sextants of the rotor position, as is known in the prior art.
FIG. 1A is an illustration of a wye-connected motor 30, as is known in the prior art. The wye-connected motor 30 in this illustration has a single-pole pair permanent magnet rotor 32 positioned such that its south pole 34 is proximate to the winding of the U-phase 36. Under these conditions, it is obvious to one skilled in the art that the W-phase 38 and the V-phase 40 are the appropriate phase pair to drive in order to initiate rotation of the rotor 32. Hence, the power stage 16 connects the W-phase 38 to Vpwr and the V-phase 40 to ground 24 resulting in current flow into the W-phase 38 and exiting the V-phase 40, as represented with the current arrows. A net effect of current flowing through coils W-phase 38 and V-phase 40 as shown in FIG. 2 is the formation of an electromagnet having a north pole at the W-phase 38 and a south pole at V-phase 40. This electromagnet produces a repulsive force between permanent magnet N-pole 42 and the electromagnet N-pole formed at the W-phase 38 and an attractive force between permanent magnet N-pole 42 and the electromagnet S-pole formed at the V-phase 38.
As N- and S-poles are attracted to each other, if the electromagnet persisted long enough in this current flow configuration, the resulting torque will move the permanent magnet N-pole 42 to a position shortly after the V-phase 40 and the permanent magnet S-pole 34 to a position shortly before the W-phase and rotation of the permanent magnet rotor 32 would stop. To perpetuate rotation of the permanent magnet rotor 32, the power stage 16 must commutate to a new phase pair. The optimum commutation point is a function of the rotor position relative to the coil of the undriven phase (the phase not driven by Vpwr). In FIG. 2, the U-phase 36 is the undriven phase. Ideally, the rotor angle would span −30° to +30° with respect to alignment with the coil of the undriven phase. As this 60° span is one sixth of one electrical revolution, it is commonly referred to as one sextant.
FIG. 1B is a 6-step commutation process further defined by Table 1, as is known in the prior art. Given the conditions illustrated in FIG. 1A, a high level description of the sequence of steps commonly referred to as 6-step commutation process is outlined in Table 1 and further illustrated in FIG. 1B.
Table 1. Six-step commutation sequence for a wye-connected motor shown in
FIG. 1ASe-DrivenquencePhaseN-pole positionS-pole positionRotorStepPairrelative to phasesrelative to phasesAngle0WVW + 30° to V − 30°U − 30° to U + 30°1.25-1.751WUV − 30° to V + 30°U + 30° to W − 30°1.75-2.252VUV + 30° to U − 30°W − 30° to W + 30°2.25-2.753VWU − 30° to U + 30°W + 30° to V − 30°2.75-0.254UWU + 30° to W − 30°V − 30° to V + 30°0.25-0.755UVW − 30° to W + 30°V + 30° to U − 30°0.75-1.25
The 6-step commutation sequence results in one electrical revolution. Given this simplified example, it is understood that a properly driven permanent magnet rotor will be driven one mechanical revolution when this six-step process is complete. An increase in number of pole pair results in an equivalent increase in the number of electrical revolutions per mechanical revolution. Comparing Table 1 and FIG. 1A, it is understood that FIG. 1A illustrates Sequence Step 0 with the permanent magnet N-pole 42 pushed from the W-phase 38 and pulled by the attraction to the V-phase 40. When the permanent magnet S-pole 34 reaches the U+30° position, the power stage 16 commutates to Sequence Step 1 driving current from the W-phase 38 to the U-phase 36 causing the U-phase to become the electromagnetic S-pole. Thus, the U-phase 36 repels or pushes the permanent magnet S-pole 34 and the W-phase 38 attracts the S-pole, continuing the clockwise motion of the permanent magnet rotor 32. The sextant parity of Sequence Steps 0, 2, and 4 is even and the sextant parity of Sequence Steps 1, 3, and 5 is odd.
Sensors in a sensored system make it easy to determine rotor position, but increase cost and provide additional pieces that can break or wear down, adding durability and reliability issues. Sensorless systems will instead measure signals on the power connections to determine rotation and speed. Sensorless systems work well controlling motors at higher speeds (e.g., revolutions per minute (“RPM”)), but often suffer control issues at low speeds, resulting in a low speed performance inferior to sensored brushless motors.
There are several well-known methods for driving a three-phase brushless direct current (DC) or permanent magnet alternating current (AC) motor without using hall-effect sensors, optical sensors, or resolvers. Several known methods for determining the position of the rotor can be used to determine which sextant to drive. Once the initial position is known, the problem remains of how to determine when to commutate to the next sextant. The aforementioned sensorless methods rely on the back electro-motive force (BEMF) signal that the motor generates to determine when to commutate the motor for continuous torque and smooth rotation. One example is a BEMF zero crossing technique, where the time measured from commutation to the point where the voltage on the undriven phase crosses a reference level or threshold is then used to determine commutation for that sextant.
However, the BEMF signal on which these control schemes rely is proportional to the speed of the motor. Consequently, at standstill or very low speed, the BEMF signal is too small to be used to determine when to commutate the motor. Thus, sensorless motors have control issues at low motor speeds.
One solution to the standstill/low speed issues is to use the voltage difference between the energizing phase and the de-energizing phase of the pulse width modulation signal that appears on the undriven phase. This demodulated undriven winding voltage signal can be used to derive the commutation point of the rotor. FIGS. 2A, 2B, and 2C illustrate the underpinning of this solution. FIG. 2A is an illustration of the rotating rotor position 100 of a motor over time, as is known in the prior art. The rotor rotates 360 degrees in a single electrical revolution and a single sextant is ⅙ of an electrical revolution or 60 degrees. Thus, FIG. 2A illustrates the rotor position 100 over the time required for the rotor to rotate through one sextant.
FIG. 2B is an illustration of the demodulated undriven winding voltage signal 104 over the same time period illustrated in FIG. 2A if the sextant parity is even, as is known in the prior art. An even sextant is one in which the demodulated undriven winding voltage increases as the rotor moves forward from the beginning of the sextant to the end the sextant while driven windings maintain a relatively constant current, as in FIG. 2B. The forward commutation level is the threshold at which the motor drive should advance to the next sextant and the backward commutation level is the threshold at which the motor drive should retreat to the previous sextant (as can happen when trying to overcome significant torque in the opposite direction of the desired direction of movement). The demodulated undriven voltage signal 104 is a good approximation of the expected signal when torque impeding rotation of the rotor and current flowing in the motor are near zero in an even sextant.
FIG. 2C is an illustration of the demodulated undriven winding voltage signal 108 over the same time period illustrated in FIG. 2A if the sextant parity is odd, as is known in the prior art. An odd sextant is one in which the demodulated undriven winding voltage decreases as the rotor moves forward from the beginning of the sextant to the end while driven windings maintain a relatively constant current, as in FIG. 2C. The forward commutation level is the threshold at which the motor is expected to advance to the next sextant and the backward commutation level is the threshold at which the motor would be expected to retreat to the previous. The demodulated undriven voltage signal 108 is a good approximation of the expected signal when torque impeding rotation of the rotor and current flowing in the motor are near zero in an odd sextant.
The demodulated undriven voltage signal 104, 108 gives only the relative position of the rotor and not the absolute position. Distortion in the demodulated undriven voltage signal 104, 108 can cause the system to miss or fail to recognize the commutation point. Distortion is introduced to the demodulated undriven voltage signal 104, 108 when a materially greater than zero current drives the motor. As illustrated in FIG. 2B, a distorted demodulated undriven voltage signal 106 is shifted by a positive magnitude relative to the undistorted signal 104. As can be observed along the distorted demodulated undriven voltage signal 106, distortion can cause distorted signal 106 to prematurely reach the forward commutation level leading to the premature commutation to the next sextant. Similarly, distortion can inhibit the distorted signal 106 from reaching the backward commutation level, which can cause the motor controller to fail to commutate to the prior sextant. Looking to the distorted demodulated undriven voltage signal 110 in FIG. 2C, illustrates a negative magnitude shift relative to the undistorted signal 108 that can similarly lead to premature commutation to the next sextant and failure to commutate to the prior sextant. If a backward commutation point is missed, then it is possible for the controller to start driving the motor backwards unintentionally, which can damage the equipment and/or user utilizing the motor. It is paramount a missed commutation point be detected and corrected.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.